The present invention generally relates to rate control methods for wireless communication systems.
In code division multiple access (CDMA) networks, the mobile stations share a reverse link channel and may transmit simultaneously on the reverse link channel. During transmission, each mobile station spreads its transmitted signal with a spreading code selected from a set of mutually orthogonal spreading codes. The base station is able to separate the signals received from the mobile stations by a correlation process. For example, if the base station desires to receive the signal transmitted by mobile station A, the base station correlates the received signal with the spreading code used by mobile station A to despread the signal from mobile station A. All other signals will appear as noise due to lack of correlation. The base station can despread signals from all other mobile stations in the same manner.
CDMA networks are interference-limited systems. Since all mobile stations operate at the same frequency, internal interference generated within the network plays a critical role in determining system capacity and signal quality. The transmit power from each mobile station contributes to the load at the base station and needs to be controlled to limit interference while maintaining desired performance objectives, e.g., bit error rate (BER), frame error rate (FER), capacity, dropped-call rate, coverage, etc. If the load is allowed to get too high, widespread outages may occur. An outage is considered to occur when the power required to maintain minimum signal quality standards is greater than the maximum transmit power of the mobile station.
Rate control is one technique used to control the load at a base station in a CDMA network. In general, the transmit power required to maintain a desired signal quality increases as the data transmission rate increases, and decreases as the data transmission rate decreases. When a mobile station is commanded to transmit at a particular data rate, the mobile station will transmit at the minimum power level needed to maintain acceptable signal quality standards. Thus, one way of controlling the load at the base station is to dynamically adjust the data transmission rates of the mobile stations.
Two well-known rate control techniques are common rate control and dedicated rate control. With common rate control and dedicated rate control, all mobile stations that need to transmit data in the reverse link are allowed to do so. Each mobile station initially begins transmitting at a specified minimum rate (sometimes called the autonomous rate) and then, depending on load of the base stations in its active set, is allowed to vary its transmission rate. The base stations periodically estimate the reverse link load and send rate control commands to the mobile stations. In common power control, a single rate control command is broadcast on a forward common power control channel and all mobile stations respond to the same rate control command. The rate control command typically comprises rate control bits, sometimes called reverse activity bits, that indicate to the mobile station 100 the link load experienced by the base station 36. In dedicated rate control, separate rate control bits are sent to each mobile station. In both common rate control and dedicated rate control systems, the mobile stations process the rate control bits and determine whether to increase, decrease or hold their current data transmission rates. When the mobile station changes its data transmission rate, it typically increases or decreases its data transmission rate to the next higher or next lower rate level defined by the applicable standard. Thus, the rate change is relative to the current data transmission rate of the mobile station.